gfs2: setattr_chown: Add missing initialization

[ Upstream commit 2d8d7990619878a848b1d916c2f936d3012ee17d ]

Add a missing initialization of variable ap in setattr_chown().
Without, chown() may be able to bypass quotas.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 686ef69ca191dcba8d325334c65a04a2589383e6)
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>